The sizes of wind-generated coastal polynyas have been observed to be nearly constant for steady atmospheric conditions owing to the balance between the advection of sea ice away from the coast and the area-averaged production rate of new ice. A simple model is used to explore the relationship of several environmental parameters to the maximum size attained by the polynya and the speed at which the maximum is reached for a given atmospheric event. The model results suggest that size is strongly a function of air temperature, such that colder air produces a smaller polynya for a given offshore wind velocity. However, size is only moderately a function of wind speed, especially for winds greater than 10 m s−1, since increasing the speed increases both the advection rate and the ice production rate. The model results are compared to observations made around a coastal polynya during February 1982 and 1983 along the southern coast of St. Lawrence Island in the northern Bering Sea and during February 1985 along the southern coast of the Seward Peninsula. The model correctly predicts the general maximum dimensions of these winter polynyas, although the atmospheric stationarity assumptions limit the usefulness of the predictions of the speed at which the maximum is reached. The results of this study suggest that the contribution of heat from the coastal ocean to the high-latitude winter atmosphere is a self-limiting process proportional to the amount of time the wind-driven ice drift has a component normal to the coast. This has important implications for the interpretation of satellite imagery for ice-covered oceans and for understanding high-latitude climate dynamics.

The size of wind‐driven coastal polynyas

This paper estimates the air/sea drag coefficient for first-year ice from recent aircraft measurements and reconciles the range of observed drag coefficients (103CD = 1.2–3.7 referenced to 10 m) for all sea ice types, based on ice roughness and seasonal meteorology. For the purpose of sea ice modeling, it is necessary to define an effective drag coefficient which relates regional stress to regional wind, because sea ice is heterogeneous on scales less than 20 km. Regional stress is influenced by the distribution of surface roughness, the buoyancy flux from quasi-periodic leads, and external atmospheric conditions, principally the inversion height. 103CD is 1.3–1.5 for smooth ice but is much greater for nonflat surfaces. For wind speeds greater than 5 m/s and air temperatures below freezing, the effective 103CD is 2.5–3.0 for nearly continuous pack ice, such as first-year ice in seasonal ice zones and central Arctic basin. The range of values of 103CD is 3.0–3.7 for unstable boundary layers typical of off-ice winds in the marginal ice zone (MIZ) or even greater if the ice has been broken by a recent storm. CD values at the lower end of these ranges are associated with low inversion heights. These coefficients are confirmed by 103CD of 2.9, 2.5, and 3.1 for first-year sea ice calculated from gust probe data collected by the NOAA P-3 aircraft, interior to the inner MIZ in the Bering Sea during the Marginal Ice Zone Experiment (MIZEX-West) in February 1983, and 103CD of 2.6 for the Arctic calculated from Arctic Ice Dynamics Joint Experiment (AIDJEX) aircraft data for February 1976. The effective drag coefficient with the presence of even a small concentration of sea ice is greater than the oceanic value as shown by a 103CD of 2.2 calculated from NOAA P-3 gust probe data over a 40-km track of 0.4 ice concentration in the outer MIZ of the Greenland Sea in June 1984 during MIZEX-84. The relation of surface wind and stress to the geostrophic wind for shallow inversion heights, typical of high latitudes, is reviewed with a turbulent closure atmospheric boundary layer model. The winter Arctic is typified by low inversions (<100 m), low geostrophic drag coefficients, and large geostrophic/surface wind turning angles, with low functional dependence on surface roughness. Seasonal ice zones are typified by a more modest influence of the inversion on boundary layer characteristics.

Atmospheric boundary layer structure and drag coefficients over sea ice

The ocean circulation on the Bering and Chukchi sea shelves is investigated using a barotropic numerical model. In the presence of the winter seasonal wind stress from the northeast, observed northward transport through Bering Strait of 0.6 Sv for the 8-month winter season 1981–1982 can be driven by a sea level difference of 0.4 m between the Pacific and Arctic oceans. This sea level difference between basins is in agreement with hydrographic estimates. In the absence of wind stress a 0.4-m sea level difference drives a northward transport through Bering Strait of 1.1 Sv. Maximum transport through Bering Strait is geostrophically limited, not frictionally or inertially limited. Major qualitative circulation features of the region are evident in the simulations. Westward intensification of the northward transport across the Bering Sea shelf is confirmed, with the major transport occurring in the relatively deep Gulf of Anadyr. In the absence of wind, Anadyr Strait contributes 72% of the northward transport through Bering Strait. With northeasterly winds there is a net clockwise circulation around Saint Lawrence Island. Within the 20-m isobath, Norton and Kotzebue sounds are little influenced by the general circulation. Northward transport in the Chukchi Sea bifurcates in the vicinity of Point Hope. The predominant branch is directed toward the northwest in a broad canyon feature south of Herald Shoal, and the balance becomes the Alaskan Coastal Current between Cape Lisburne and Point Barrow.

Northward flow in the Bering and Chukchi seas

This study is aimed at modeling mesoscale processes such as upwelling/downwelling and ice edge eddies in the marginal ice zones. A two-dimensional coupled ice-ocean model is used for the study. The ice model is coupled to the reduced gravity ocean model through interfacial stresses. The parameters of the ocean model were chosen so that the dynamics would be nonlinear. The model was tested by studying the dynamics of upwelling. Wings parallel to the ice edge with the ice on the right produce upwelling because the air-ice momentum flux is much greater than air-ocean momentum flux; thus the Ekman transport is greater than the ice than in the open water. The stability of the upwelling and downwelling jets is discussed. The downwelling jet is found to be far more unstable than the upwelling jet because the upwelling jet is stabilized by the divergence. The constant wind field exerted on a varying ice cover will generate vorticity leading to enhanced upwelling/downwelling regions, i.e., wind-forced vortices. Steepening and strengthening of vortices are provided by the nonlinear terms. When forcing is time-varying, the advection terms will also redistribute the vorticity. The wind reversals will separate the vortices from the ice edge, so that the upwelling enhancements are pushed to the open ocean and the downwelling enhancements are pushed underneath the ice.

Coupled ice-ocean dynamics in the marginal ice zones Upwelling/downwelling and eddy generation

Using the original setup described in Gallee et al. (2013), the MAR regional climate model including a coupled snowpack/aeolian snow transport parameterization, was run at a fine spatial (5 km horizontal and 2 m vertical) resolution over 1 summer month in coastal Adelie Land. Different types of feedback were taken into account in MAR including drag partitioning caused by surface roughness elements. Model outputs are compared with observations made at two coastal locations, D17 and D47, situated respectively 10 and 100 km inland. Wind speed was correctly simulated with positive values of the Nash test (0.60 for D17 and 0.37 for D47) but wind velocities above 10 m s −1 were underestimated at both D17 and D47; at D47, the model consistently underestimated wind velocity by 2 m s −1. Aeolian snow transport events were correctly reproduced with the right timing and a good temporal resolution at both locations except when the maximum particle height was less than 1 m. The threshold friction velocity, evaluated only at D17 for a 7-day period without snowfall, was overestimated. The simulated aeolian snow mass fluxes between 0 and 2 m at D47 displayed the same variations but were underestimated compared to the second-generation FlowCapt ™ values, as was the simulated relative humidity at 2 m above the surface. As a result, MAR underestimated the total aeolian horizontal snow transport for the first 2 m above the ground by a factor of 10 compared to estimations by the second-generation FlowCapt ™. The simulation was significantly improved at D47 if a 1-order decrease in the magnitude of z 0 was accounted for, but agreement with observations was reduced at D17. Our results suggest that z 0 may vary regionally depending on snowpack properties, which are involved in different types of feedback between aeolian transport of snow and z 0 .

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Comparison between observed and simulated aeolian snow mass fluxes in Adélie Land, East Antarctica

Turbulent flux measurements were made at four levels (42, 90, 195, and 340 m) by the NOAA P-3 aircraft over first-year sea ice in the northern Bering Sea during February 1982. Three profiles of momentum flux and mean wind were used to calculate an air-ice drag coefficient CD of 3.0 ± .6 × 10−3 referenced to a 10-m anemometer height. The boundary layer was slightly unstable (zi/L = −1.2, where zi, was the inversion height of 660 m and L the Monin-Obukhov length). The mean wind speed at the 42-m height was 17 m/s, and the air temperature was −20°C. From turbulent heat flux measurements the value of the bulk heat transfer coefficient CH was 0.73 ± 1.6 × 10−3, giving a CH/CD of 0.24. The Bowen ratio was greater than 2.8. Comparison of the present turbulent flux and variance profiles with those collected over the ocean shows agreement, which increases confidence in the calculations. The geostrophic drag coefficient |u*|/|G|, where u* is the friction velocity and G is the geostrophic wind, was 0.047. The turning angle of the surface wind (measured by an anemometer at a height of 3 m on the ice) was 32°, and the ratio of surface wind speed to the geostrophic wind speed was 0.76.

Richard O. Gilmer

Papers

Air‐ice drag coefficients for first‐year sea ice derived from aircraft measurements